The strength of Apple's revised encryption scheme in iOS 8 hinges on users choosing a strong passcode or password, which they rarely do, according to a Princeton University fellow.

Apple beefed up the encryption in its latest mobile operating system, protecting more sensitive data and employing more protections within hardware to make it harder to access. The new system has worried U.S. authorities, who fear it may make it more difficult to obtain data for law enforcement since Apple has no access to it.

"Users with any simple passcode have no security against a serious attacker who's able to start guessing with the help of the device's cryptographic processor," he wrote.

If an iPhone is seized when it's turned off, it's unlikely that the keys can be derived from its cryptographic co-processor called the "Secure Enclave," which does the heavy lifting to enable encryption.

But if an attacker can boot the phone and get access to the Secure Enclave, it would be possible to start guessing passwords in a brute-force attack, and that's where the weakness lies.

Apple doesn't make it easy to completely copy all of the data on a device and boot it up using external firmware or another operating system, which would be an attacker's first step, Bonneau wrote.

His theory of how easy it would be to obtain the data from a device is dependent on an attacker being able to bypass the complicated "secure boot" sequence of an iOS 8 device.

"We'll assume this can be defeated by finding a security hole, stealing Apple's key to sign alternate code or coercing Apple into doing so," he wrote.

If that is possible, the attacker can begin guessing passcodes or passwords against the Secure Enclave. Apple's documentation suggests that such guesses could be conducted at a rate of either 12 guesses per second or one guess every five seconds.

By default, Apple asks users to set a "simple passcode," which is a four-digit numerical PIN, although users can set much longer pass phrases.

If an attacker can guess four-digit passcodes at 12 per second, the entire space of 10,000 possible PINs can be guessed in about 13 minutes, or 14 hours at the slower rate of one per five seconds, Bonneau wrote.

Apple could slow down the rate at which passwords can be entered, but that would probably annoy users. An alternative would be to limit the number of overall incorrect guesses and erase the phone's data, but that approach would require warning users that they're at risk of blanking their phone if they continue guessing, he wrote.

Even users who opt to set a longer passcode or phrase rather than a four-digit PIN are probably still at risk.

Bonneau said it's unlikely that users choose stronger passwords to protect their devices than Web services accounts, since "entering passwords on a touchscreen is painful."

The best advice is to create a password that is at least a 12-digit random number or a nine-character string of lower-case letters, he wrote. And do not use that password for any other services.

"These aren't trivial to memorize, but the vast majority of humans can do this with practice," Bonneau wrote.

If there's a fear a device may be seized, it's best to keep it off -- such as when crossing international borders -- as that offers the greatest level of encryption protection, he wrote.